The present invention relates to a planar X-ray detector for a medical X-ray diagnostic apparatus.
In recent years, in the medical field, database are being prepared with respect to the medical data of patients in order to perform medical treatments promptly and exactly. This is because, since a patient often utilizes two or more medical institutions, a medical institution requires the data on the patient prepared in another medical institution in order to perform an exact medical treatment.
There are also demands for preparing database on image data of the radiographs, and therefore it is requested to digitize the radiograph images in accordance with the demands. In the medical X-ray diagnostic apparatus, it has been customary to photograph an image using a silver halide film. In order to digitize such an image, it is necessary to develop the photographed image, followed by reading the image with a scanner or the like, leading to requirement of much labor and time.
Recently, an image intensifier TV system of directly detecting a digital image by using a photoelectron emission film, a vacuum tube, a phosphor film, and a CCD camera of a size of about one inch has been realized. However, in the diagnosis of, for example, lungs, a large area of about 40 cmxc3x9740 cm is photographed, making it necessary to use an optical apparatus for converging light. As a result, a large volume size of the apparatus becomes a serious problem.
As a system capable of solving the problems inherent in the above two systems, proposed is a planar X-ray detector of a direct conversion type using an amorphous silicon thin film transistor (a-Si TFT). In the planar X-ray detector, an incident X-ray is directly converted into electric charges by an X-ray charge conversion film in each pixel.
FIG. 1 shows the element structure of one pixel of the conventional planar X-ray detector. A capacitor (Cst) of a laminate structure consisting of a capacitor electrode 404, an insulating layer 103 and an auxiliary electrode 502, and a switching TFT 402 and a protective TFT 411, each of which is connected to the capacitor, are formed on a glass substrate 101. A passivation film is formed on these members, having a contact hole. Formed on the passivation film is a laminate structure consisting of a pixel electrode 503, which is connected to the auxiliary electrode 502, a p-type contact film 601, an X-ray charge conversion film 403, an n-type contact film 602, and a common electrode 603. The pixels are arranged to form an array.
When an X-ray is incident on the planar X-ray detector, the X-ray is converted into electric charges by the X-ray charge conversion film 403, and the electric charges are accelerated by an electric field applied between the common electrode 603 and the pixel electrode 503 and then accumulated in the capacitor. The switching TFT 402 is driven through a scanning line so as to transport the charges accumulated in the capacitor to a signal line and to an amplifier circuit 410. The protective TFT 411 functions to release charges when excessive charges are generated.
Conventionally, a film of Se, PbI2 or the like is mainly used as the X-ray charge conversion film 403. On the surface of the planar X-ray detector, the X-ray charge conversion film 403 is covered with the n-type contact film 602 and the common electrode 603. However, the X-ray charge conversion film 403 is exposed to the outside in the side surface of the planar X-ray detector. Since the material such as Se and PbI2 gives a detrimental effect to the human body, it is undesirable for the X-ray charge conversion film 403 to be exposed to the outside. Also, in order to obtain a high-precision image, it is necessary to form the X-ray charge conversion film 403 uniformly. However, it is difficult to form a large area uniform film by using a metal or a metal compound such as Se and PbI2.
Japanese Patent Publication (Kokoku) No. 4-63555 discloses an X-ray sensor comprising an X-ray detecting layer formed of phosphor particles and photoconductor particles dispersed in the transparent charge transport material. The X-ray sensor is applied to an X-ray CT scanner. In the X-ray sensor, the phosphor particles irradiated with the X-ray emits fluorescent light, and the photoconductor particles irradiated with the fluorescent light generates electrons and holes, which are transported by the charge transport material. However, where the X-ray sensor is applied to a planar X-ray detector, a problem is brought about that the generated fluorescent light spreads in an isotropic manner and arrives at adjacent pixels so as to bring about crosstalk and deterioration of spatial resolution of detected image.
Japanese Patent Disclosure (Kokai) No. 3-273687 discloses a radiation absorbing material prepared by dispersing in an organic semiconductor such as metal phthalocyanine or an inorganic semiconductor of such as CdS, a metal of such as W or Pb element having an atomic number larger than that of the basal organic semiconductor or inorganic semiconductor. However, since the prior art uses a metal as the radiation absorbing material, electron-hole pairs have high probability to recombine within the metal, because there exists a large Coulombic attractive force between small distance electron-hole pair due to small velocity caused by small electric field in metal having a high conductivity, making it difficult to take out the charges to the outside, and leading to poor efficiency.
An object of the present invention is to provide a planar X-ray detector that enables to effectively take out charges generated by an incident X-ray to the outside.
According to an aspect of the present invention, there is provided a planar X-ray detector, comprising: a pair of electrodes, and an X-ray charge conversion film disposed between the electrodes, the film containing a mixture of inorganic semiconductor particles sensitive to an X-ray and a carrier transport material.
According to another aspect of the present invention, there is provided a planar X-ray detector, comprising: an X-ray charge conversion film converting an incident X-ray into electric charges; pixel electrodes provided on the X-ray charge conversion film corresponding to respective pixels arranged in an array; switching elements connected to the respective pixel electrodes; signal lines, each of which is connected to a column of switching elements; scanning lines, each of which transmits driving signals to a raw of switching elements; and a common electrode provided on the surface of the X-ray charge conversion film opposite to the surface on which the pixel electrodes are provided, wherein the X-ray charge conversion film contains an X-ray sensitive material comprising inorganic semiconductor particles, and a carrier transport material comprising an organic semiconductor.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.